This invention relates to an improved method for making GeO.sub.2 -doped glass articles. A specific application of the invention is the production of optical waveguide fibers and, in particular, preforms from which such fibers can be produced.
Optical waveguide fibers consist of a core surrounded by cladding material having a refractive index lower that that of the core. Depending on the type of fiber and its desired performance characteristics, the radial distribution of the refractive index across the face of the fiber can be simple or complex. For example, single-mode fibers typically have a refractive index profile which is a simple step, i.e., a substantially uniform refractive index within the core and a sharp decrease in refractive index at the core-cladding interface. On the other hand, to produce a high bandwidth, multimode fiber requires achieving a nearly parabolic radial refractive index profile in the fiber core so as to minimize intermodal dispersion.
Optical fibers can be prepared by various known techniques. The present invention is concerned with those techniques such as the outside vapor deposition (OVD) technique and the axial vapor deposition (AVD) technique wherein a porous glass preform is formed and then consolidated.
Preforms produced by vapor deposition techniques typically are composed of silicon dioxide (SiO.sub.2) selectively doped with at least one metal or metalloid oxide to provide the desired refractive index profile. The preferred dopant in commerical use today is germanium dioxide (GeO.sub.2). In processes for manufacturing optical fibers, precursors for the deposition of metal oxide dopants are relatively expensive raw ingredients. It is therefore important that the dopant be effectively incorporated in the preform with a minimum of dopant loss during processing.
In accordance with the OVD technique, which will be discussed during a description of the invention, glass particles can be formed by oxidizing and/or hydrolyzing the halide materials SiCl.sub.4 and GeCl.sub.4 in a burner. The preform is formed from the glass particles by moving the burner back and forth along the length of a rotating mandrel. See U.S. Pat. No. 4,486,212, for example. The distance between the mandrel and the burner is selected so that the glass particles collect on the mandrel in thin layers with each pass of the burner. The amount of halide materials supplied to the burner is adjusted during the glass laydown process so as to produce a dopant concentration in the preform which varies with radius. This dopant concentration profile is selected so that the finished fiber will have the desired refractive index profile.
The mandrel is removed from the porous preform, thereby forming an aperture. The porous preform is then placed in a consolidation furnace where it is dried and sintered. During the drying step or during the entire consolidation process, depending upon the particular consolidation process employed, a first drying gas mixture, which usually contains helium and a drying agent such as chlorine or fluorine, flows into the aperture. A drying agent can also be flowed through the furnace (see, for example, U.S. Pat. No. 4,165,223). The drying step reduces the residual OH content of the preform, thereby reducing in the resultant optical fiber the absorption loss caused by OH groups in the vicinity of the 1300 nm operating wavelength. The step of sintering a porous preform produces a dense, substantially clear glass article which itself can be drawn into the optical fiber or which can be provided with additional cladding and then drawn into an optical fiber. The entire porous preform can be dried before the sinter step begins; alternatively, the preform can be subjected to a gradient consolidation process whereby the temperature of each individual element of the preform increases and decreases with the approach and passing of the hot zone, respectively. As the hot zone approaches, the preform element becomes sufficiently hot that the drying gas mixture can react with the OH ions in the glass, but the preform temperature is not so high that preform porosity is decreased to the point that drying gas flow is impeded. As the preform element is subjected to the maximum temperature region of the hot zone, the pore size decreases and the preform element then completely sinters and clarifies.
During the consolidation process, dopant from the core portion of a porous preform can migrate through the pores to the cladding portion, thereby creating a dopant depleted region at the edge of the core and a corresponding dopant rich region in the adjacent cladding; this combination is known as a "diffusion tail". Moreover, in a multimode fiber wherein a central region of the core has a higher dopant concentration than an adjacent region of greater radius, dopant can migrate from the region of higher concentration to the region of lesser concentration to alter the core refractive index profile.
Two features of the refractive index profile, the central dip and the diffusion tail, have been recognized as limiting the optical performance of optical fibers. The central dip has been shown to be correlated with decreasing the optical bandwidth of the fiber. Modelling has revealed that the diffusion tail has the effect of increasing the optical attenuation of the fiber. Moreover, the migration of germanium out of the preform is very costly, especially in a process for fabricating multimode optical fibers. A multimode fiber fabrication process requires a large amount of germanium source material in order to produce cores having greater radii and greater refractive indices as compared to single-mode fibers. Therefore, processes which could retain more germania in the sample could result in the production of more product (increased select), a better performance distribution, and a capital avoidance of buying germanium-containing source materials.